1. Field of the Invention
This invention relates to free electron lasers (FELs), and more particularly to FELs having a controllable phase and frequency for steering an output beam.
2. Description of the Related Art
An FEL amplifies short-wavelength radiation by stimulated emission, using a beam of relativistic electrons. The electrons are not truly "free", since they are under the influence of magnetic forces which cause them to radiate, but they are "free" in the sense that they are not bound into atoms as in the case of a conventional laser. The FEL radiation is usually caused by passing the electrons down a magnetic device known as an undulator or "wiggler", in which the electrons are forced to execute a periodic oscillatory trajectory in space. The wiggler may be a helical field produced by a bifilar helical winding, a linearly polarized field made by a set of alternating polarity magnets, an electrostatic device, or an electromagnetic wave field. The travelling electrons see an oscillating field except at certain regions where they bunch together and travel in synchronism. This causes them to radiate coherently and release appreciable amounts of power. A recent text on this type of device is Thomas C. Marshall, "Free-Electron Lasers", MacMillan Publishing Company, 1985.
FIG. 1 illustrates one conceptual application for the FEL. An array of FEL voltage power supply 4. A stabilized FEL oscillator 6 operates under the control of a frequency synthesizer and controller 8 to produce a local oscillator signal which turns on the FEL amplifiers 2 at a particular phase and frequency. To provide a phased array, phase shifters 10 are provided in front of each FEL amplifier 2 to provide unique phase shifts. The FEL amplifier outputs are directed onto an antenna array 12 which radiates the array of signals as a radar beam 14. The angle (direction) of beam 14 is controlled by the relative phases of the signals emitted from the various FEL amplifiers, while the divergence or width of beam 14 is controlled by the FEL frequencies. By controlling the phase shifters 10 such that each successive FEL amplifier in the array has a progressively greater or smaller phase shift, the output wavefronts from each FEL amplifier in succession will be slightly delayed or retarded with respect to the wavefront for the previous FEL. This causes the beam 14 to be transmitted at an angle, in a manner analogous to the change in angle of an optical beam transmitted at an angle between materials of different refractive indices.
A principal problem in implementing this type of phased array is that currently known phase shifters 10 are relatively narrow band, and cannot be used over a wide portion of the electromagnetic spectrum. Waveguides cannot be used for phase shifting, since their diameters are normally in the quarter wavelength range; this is too small for FELs, which are typically many wavelengths in diameter. Accordingly, phase shifters which have been proposed are large area optical materials that produce a phase shift at a given wavelength. A different phase shifting material is needed at each different frequency band, and for sub-mm wavelength signals (infrared) it is difficult to find transparent materials with the desired optical properties. Another representative application for phase shifting is to encode the waveform during a pulse in order to perform accurate radar measurements. Generally, radar systems require an ability to rapidly alter the phase of the signal by 180.degree. or to induce a chirp or frequency change by continuously adjusting the phase.